1. Field of the Invention
The present invention relates to a polymer electrolyte fuel cell.
2. Description of Related Art
In order to obtain a fuel cell with a high power density, it is efficient that components and/or regions in the fuel cell without direct contribution to power generation are trimmed down as possible so as to reduce the entire volume of the fuel cell. It is considered that reducing the amount of coolant in a fuel cell is one of possible means to satisfy above requirement. As described in JP-A-2005-235727, there is a method to reduce the amount of coolant by utilizing a latent heat cooling instead of a sensible heat cooling. The latent heat cooling utilizes the heat of vaporization of coolant, and the sensible heat cooling utilizes the specific heat of coolant. In the latent heat cooling, 2% of the amount of coolant used in the sensible heat cooling is enough theoretically to obtain a comparable cooling effect.
For effective latent heat cooling, it makes the coolant be fine droplets to increase a surface area per unit volume of the coolant so as to facilitate vaporization. Moreover, the generated fine droplets of coolant are carried to a cooling place and made to hit against a member to be cooled. A gas can carry the fine droplets of coolant because the settling rate of the fine droplets is extremely low in the gas. However, at a bent portion of a pipe or another place where the acceleration works on a fluid to bend, the fine droplets of coolant are easy to separate from the carrier gas, thereby hitting against the pipe wall. As a result, there is a problem that the coolant is wasted at places where cooling is not needed.
Furthermore, the reaction gas before a power generation reaction in a fuel cell usually includes water vapor. The vapor pressure is already near the saturated vapor pressure. Thus, even though the fine droplets of coolant are added to the reaction gas, it is difficult to vaporize due to the saturated vapor pressure, thereby decreasing the cooling effect. In addition, there is a problem that excessive moisture makes it difficult to remove water generated by the cell reaction.
By the way, in a fuel cell, hydrogen is used as an anode gas (referred to below as the AN gas) and oxygen in the air is used as a cathode gas (referred to below as the CA gas). These gases are consumed as in a chemical equation below, generating water, heat and electric power.2H2+O2→2H2O+(heat)+(electric power)
This reaction occurs during a flow from the upstream to the downstream along the flow fields in a fuel cell, so the reaction gas is consumed as the flow proceeds. While, water vapor generated by a cell reaction flows into the CA gas; and also water flows to the AN gas side on the basis of diffusion and electroosmosis, thereby increasing the water vapor concentration in the both sides (cathode and anode). When the water vapor exceeds the saturated concentration, condensed water generates and is easy to block the flow field. This condensed water then causes insufficiency of the reaction gas and flooding, thereby lowering the cell voltage and shortening a life of the cell.
Furthermore, the condensed water releases latent heat and thereby a temperature of the fuel cell rises locally (uneven temperature distribution occurs). In order to protect the membrane electrode assembly (MEA), however, the maximum temperature must be limited; therefore fuel utilization ratio should be controlled to lower. As a result, the output of the entire cell decreases.
On the contrary, in order to provide a fuel cell with a high power density and a low cost, it is desirable to raise the fuel utilization ratio to almost 100%. It is also desirable to reduce the regions such as manifolds of a separator and other parts without direct contribution to power generation so as to reduce the entire volume. In order to achieve above conditions, it is necessary that cooling performance is improved and that a uniform temperature distribution is attained. Moreover, it is also important to suppress a flooding phenomenon.